I said you don’t need a particle に ( = ni) with 今日 (= kyou) today, 昨日 ( = kinou) yesterday , 明日 ( = ashita) tomorrow, but the confusing part is we use a particle, は ( = wa) with them.

* は ( = wa): A subject marker emphasizing the mentioned time / to show contrast. (Maybe not contrasting one time with another but rather the mentioned time with unmentioned times. See the examples below.)

X 今日、いい天気です。(wrong)

= Kyou, iitenki desu.

↓

Ex. 今日は、いい天気です。

= Kyou wa, ii tenki desu.

= (Today is nice weather.)

= It is nice today.

The weather changes everyday, but (at least) today is nice.

****

X 月曜日、忙しいです。(wrong)

= Getsuyoubi, isogashii desu.

↓

Ex. 月曜日は忙しいです。

= Getsuyoubi wa isogashii desu.

(= Mondays are busy days.)

= I am busy on Mondays.

***

Ex. 去年の今頃はシンガポールにいました。

= Kyonen no imagoro wa shinagpooru ni imashita.

= I was in Singapore around this time last year.

So you can simply emphasize a certain period of time, a certain day, a certain month, a certain week, or a certain year by using は ( = wa). Or you can show contrast when you do something special at a particular time period.

From the picture above:

Flama「明日は近所の犬と合コンがあるんだよ。」

= Ashita wa kinjo no inu to goukon ga arundayo.

= You know I have a group date with neighborhood dogs tomorrow.

Note: You can omit は ( = wa).

By adding は ( = wa) , you can express tomorrow is a special day or doing something different on that day.

Ex. 今日、学校に行きます。

= Kyou, gakkou ni ikimasu.

= I will go to school today.

(regular statement)

↓Emphasizing 今日( = kyou) today

今日は、学校に行きます。

= Kyou wa, gakkou ni ikimasu.

Ex. 今日、学校は休みです。

= Kyou, gakkou wa yasumi desu.

= There are no classes at school today.

(regular statement)

Ex. 今日は学校は休みです。

= Kyou wa gakkou wa yasumi desu.

= There are no classes at school today.

(You usually have a class on other days but not today.)

Ex. 昨日、買い物に行った。

= Kinou, kaimono ni itta.

= I went shopping yesterday.

(regular statement)

↓Emphasizing 昨日 ( = kinou) yesterday

昨日は買い物に行った。

= Kinou wa kaimono ni itta.

Ex. 明日、釣りに行きます。

= Ashita , tsuri ni ikimasu.

= I will go fishing tomorrow.

↓Emphasizing 明日 ( = ashita) tomorrow

Ex. 明日は、釣りに行きます。

= Ashita wa, tsuri ni ikimasu.

Ex.２週間、同じホテルに滞在します。

= Nishuukan, onaji hoteru ni taizai shimasu.

= I will stay at the same hotel for two weeks.

(regular statement)

↓ Emphasizing ２週間 (=nishuukan) two weeks. (at least two weeks.)

Ex.２週間は、同じホテルに滞在します。

= Nishuukan wa, onaji hoteru ni taizai shimasu.

Ex. 来週、会議があります。

= Raishuu, kaigi ga arimasu.

= We have a meeting next week.

(regular statement)

↓Emphasizing 来週 (= raishuu) next week

Ex. 来週は会議があります。

= Raishuu wa kaigi ga ariamsu.

Note: You can also say:

* 会議は来週です。

= Kaigi wa raishuu desu.

= The meeting will be held next week.

*には ( = niwa) / までには ( = made niwa) by: When you try to do something by a certain period of time

I think what you don’t get is the expression 今日になっても
In Japanese, when the certain time comes, there is an expression
(time related word)+ になる
明日になる
３時になる
来年になる

The literal meaning is the time (day/time/year 日付／時間／年が) turn(ed) （になる）
So this sentence’s literal meaning is
今日になってもまだ息子から連絡がない。
Although/Even though the day/date turned today, I haven’t heard anything from my son.
→I haven’t heard from my son until today.

Hi. I’ve been visiting this site for quite some time now. I just wanted to let you know how much I appreciate these lessons and how much I find them helpful!
Thank you so much for your hard work.
Sending my love from Slovenia to you and to Maggie above*

If I would have translated this sentence into Japanese, I would have written: この旅行の出発は2ヶ月前に申し込んで下さい。 So, in my sentence の would be used to modify 出発. But I don’t quite understand its function in your sentence. It kind of sounds like a relative clause, in which の replaces the が, but I don’t know. So what kind of の is 出発の２ヶ月?

2.明日から夏休みが始まります。 Can you use は after 明日 in this sentence or do you have to use から?